Devitrification-resistant coating for high-silica glasses

ABSTRACT

A devitrification-resistant glaze for high-silica glasses, particularly effective in inhibiting surface devitrification at elevated temperatures under highly alkaline conditions, formed from a coating consisting essentially of Ta2O5 but also containing up to about 85 percent by weight Al2O3 and up to about 50 percent by weight SiO2. The coating is preferably applied in the form of an aqueous slurry which is dried and fired to produce a vitreous glaze.

United States Patent [1 1 [111 3,754,980

Maimendier 1 Aug. 28, 1973 [54] DEVITRIFICATION-RESISTANT COATINGR2l,l75 8/1939 Morey 106/47 0 FOR HIGHSILICA GLASSES 3,414,465 12/1968Baak et a1 1 17/125 [75] Inventor: Joseph W. Malmendier, Painted Post,NY.

[73] Assignee: Corning Glass Works, Corning, N.Y.

[22] Filed: Aug. 6, 1971 [21] Appl. No.: 169,824

[52] US. Cl. 117/124 A, 65/30, 106/47, 106/48, 106/52, 117/46 FC,117/125 [51] Int. Cl ..C03c 17/02, C03c 5/00 [58] Field of Search 117/46PC, 123 A, 117/124 A, 125; 106/47 R, 47 A, 48, 52, 39 DV; 65/30, 33

[56] References Cited UNlTED STATES PATENTS 2,351,974 6/1944 Kollmar106/48 2,995,468 8/1961 Steiger 106/48 2,396,873 3/1946 Morrison et a1.117/124 A 3,055,762 9/1962 Hoffman 117/124 A 3,637,425 1/1972 McMillanet al.. 117/125 3,539,387 1l/197O Kelly et al 117/125 3,540,896 11/1970Flicker 117/46 FC OTHER PUBLICATIONS King et al., Some Properties ofTamale Systems," (Abstract), Nuclear Science Abstracts, Vol. 10, (1956),pp. 995-95.

Reeder et al., Ceramic Adhesive Bonding of Refractory Metals-APreliminary Investigation," Bull of Am. Ceramic Soc., Vol. 42, (1963),pp. 337-39.

Primary Examiner-William D. Martin Assistant E qrdminer-William H.Schmidt Attorney-Clarence R. Patty. J r., Kees van der Sterne et a1.

[57] ABSTRACT A devitrification-resistant glaze for high-silica glasses,particularly effective in inhibiting surface devitrification at elevatedtemperatures under highly alkaline conditions, formed from a coatingconsisting essentially of T21 0, but also containing up to about 85percent by weight A1 0 and up to about 50 percent by weight SiO Thecoating is preferably applied in the form of an aqueous slurry which isdried and fired to produce a vitreous glaze.

3 Claims, No Drawings DEVITRIFICATION-RESISTANT COATING FOR HIGH-SILICAGLASSES BACKGROUND OF THE INVENTION The coatings of the presentinvention are related to the coatings described in the co-pending U.S.Pat. application of T. H. Elmer and J. W. Malmendier, Ser. No. 169,823,filed concurrently and commonly assigned herewith. However, glazesproduced from the coatings described in the co-pending application areuseful primarily in inhibiting devitrification in highsilica glassesinduced by contact with food-ash at elevated temperatures.

This invention has applicability principally in the field of high-silicaand fused silica glasses. Devitrification of these glasses, rather thansoftening, often limits their use temperature and duration of use. Forthis reason, means for stopping or retarding surface devitrification inhigh-silica glasses have long been sought. The rates of devitrificationof the glass depend on many variables, including the amounts ofimpurities in the glass, the moisture content of the glass-makingmaterials used, the temperature of the environment, and the compositionof the ambient atmosphere. Since it is difficult to regulate thesevariables during the actual use of the material, it would be desirableto develop a glaze which could inhibit devitrification of high-silicaglasses independently of these variables under all types of conditions.Among the conditions which are known to induce devitrification inhigh-silica glasses are elevated temperatures and highly alkalineconditions, such as might be encountered in alkali metal vapor lamps orhighly alkaline solutions or atmospheres.

Earlier attempts by others to inhibit devitrification in high-silicaglasses centered on additions of ZrO,, TiO or A1 to the glass asdopants, but with inconsistent results. A very complete literaturereview is given up to the year 1957 in the thesis of S. D. Brown,entitled The Devitrification of High Temperature Glass," Ph.

D. Thesis, University of Utah (1957). This survey of 4 the literature ondevitrification was extended to 1962 by F. E. Wagstaff in Kinetics ofCrystallization of Vitreous Silica," Ph. D. Thesis, University of Utah(1962). The devitrification of high-silica glasses usually results inthe formation of cristobalite. An excellent description of the processis given by Ainslie et al., in a paper presented at the 1962 Symposiumon the Nucleation and Crystallization of Glass.

In the present work, protection from devitrification was sought by theformation of surface glazes. Glazing is easily adapted to production andnot likely to interfere with the bulk properties of a glass. Theobjective was to inhibit devitrification under the extremely adverseconditions of an alkali-rich atmosphere at elevated temperatures, e.g.,above about 700C, since it was felt that a protective glaze which wassuccessful in inhibiting devitrification under such conditions wouldalso protect the glass under other conditions tending to inducedevitrification.

SUMMARY OF THE INVENTION I have now discovered certain protective glazecompositions which are quite effective in inhibiting devitrification inhigh-silica glasses at elevated temperatures under highly alkalineconditions. These glazes consist essentially of Ta,O but may alsocontain significant quantities of M 0 and SiO;. They are formed byapplying to the glass a coating consisting essentially of the describedoxides, or of compounds thermally decomposable to yield the describedoxides in specified proportions, and then firing the coating to cause itto react with the glass to produce a vitreous glaze. The coating ispreferably applied in the form of an aqueous slurry or suspension of thedesired compounds, then dried, and finally reacted with the surface ofthe glass by heating with a gas-oxygen flame, although other methods ofdeposition such as plasma spraying, flame spraying, or vapor depositionwould also be suitable. Coatings suitable for use in producing theglazes of the invention may consist, on the oxide basis, that is, on thebasis of the composition of the oxide mixture resulting when the coatingis oxidized on firing, of about l5-l00 percent Ta,0 0-85 percent A1 0and 0-50 percent SiO by weight. Especially preferred are coatings consisting essentially, in weight percent on the oxide basis, of about55-65 percent Ta O and 35-45 percent SiO and coatings consistingessentially in weight percent on the oxide basis, of about 25-35 percentTa O and 65-75 percent A1 0 DESCRIPTION OF THE PREFERRED EMBODIMENTS Theglazes produced from the coatings of the present invention areparticularly suitable for use with fused silica and with high-silicaglasses such as reconstructed 96 percent silica glass. Examples of thelatter include Corning Code 7913 glass. These materials are presentlyused for numerous high temperature applications, such as lamp envelopes,heat sheaths for electric heating elements, and furnace window glasses,where devitrification poses a problem which can be alleviated throughthe use of the glazes of the present invention. However, the glazesofthe present invention have also been applied to glasses containing aslittle as percent silica by weight with good results.

The single essential constituent of the protective glazes of theinvention is Ta O While A1 0 has been shown to have some beneficialeffect in inhibiting devitrification when used in conjunction with Ta Ol have found that coatings consisting essentially of A1 0 alone do notprovide the devitrification resistance required for high temperatureapplications under highly alkaline conditions, and can under somecircumstances actually act as a positive catalyst for devitrification.The presence of SiO, in the glazes of the invention does not appear toimprove the protective capability thereof, but SiO is helpful in fonninga stable colloidal suspension of Tap, which is required to obtainuniform distribution of Ta,0 on the surface to be protected. Hence, Ihave found that, while glazes produced from coatings consistingessentially of Ta,O are suit able for the purpose of providingdevitrification protection according to the present invention, thecoatings may also contain up to about percent by weight A1 0 and up toabout 50 percent SiO,, provided the Ta o content does not fall belowabout 15 percent by weight. Suitable coatings, therefore, consist, inweight percent on the oxide basis, of about 15-100 percent Ta O 0-85percent Al O and 0-50 percent SiO,. Especially preferred for ease ofapplication and compatibility with high silica substrates in terms ofthermal and physical properties are coatings consisting essentially, inweight percent on the oxide basis, of about 25-35 percent Ta O and 65-75percent A1 0 and coatings consisting essentially, in weight percent onthe oxide basis, of about 55-65 percent Ta O and 3545 percent SiO Theexact compositions of the glazes produced from the desired coatings arenot readily determined, since the glazes are a reaction product of thecoatings and the glass substrate.

My preferred method of applying the coatings of the present inventioninvolves the use of a slurry or suspension of the desired oxides, orcompounds thermally decomposable to the desired oxides, in water. Thesteps of the method normally comprise preparing an aqueous suspensionconsisting essentially of a coating component and a water component,wherein the coating component consists essentially of compounds which,upon firing, will yield, in weight percent on the oxide basis, about15-100 percent Ta O -85 percent A1 0 and 0-50 percentSiO saidcoating'component comprising about 4-15 percent by weight of thesuspension, applying the suspension to the glass article to be treated,drying the suspension on the glass to form an evaporation product layer,and firing the evaporation product layer to form a vitreous protectiveglaze on the glass. While in many cases the oxides themselves willcomprise the coating component of the suspension, and thus, theevaporation product layer, other compounds may be substituted which willyield the required oxides in the desired proportions upon firing theevaporation product layer to form a vitreous glaze. Thus, I prefer touse TaCl rather than Ta O in preparing suspensions according to thepresent invention because it dissolves easily and reacts readily withthe substrate. This merely involves incorporating TaCl into thesuspension in quantities such that the fired coating will contain about-100 percent Ta O as calculated in weight percent on the oxide basis,from the proportions of the coating compounds contained in the slurry.

Insoluble solids used in making up the slurry, such as SiO and A1 0should have a particle size small enough to be both readily mixed andreasonably stable in suspension. Of course, volatile solvents other thanwater may be used to form the suspension, if desired.

I have also found it desirable, in preparing the aqueous suspensionsdescribed, to adjust the pH thereof with, for example, HCl or NI-LOH, inorder to improve the stability of the suspension. Preferably, the pH ofthe slurries prepared as described will range between about 8 and 9, asthe result of minor additions of either of these two compounds.

The preferred method for applying the slurries to the glass articles tobe protected comprises dipping the glass into the slurries. In applyingthe slurry, care must be taken not to obtain an excessively thickcoating. It has been found that there is a thermal expansion differencebetween the glazes of the invention and the highsilica base glasses towhich they will normally be applied, and crazing has been observed inexcessively thick glazes as a result of this expansion difference.Glazes ranging in thickness after firing from about 10-50 microns aresufficient to provide good devitrification protection while avoiding theproblem of crazing, particularly if the preferred compositions are used.Glazes within this thickness range are readily obtained by a single dipcoating in slurries having a solids content in the range from about 7-14percent by weight.

After applying the slurries to the glass, the moisture should be removedfrom the coating prior to firing. The drying process may be acceleratedby heating, for example, in an oven, under a heat lamp, or with a flame,if desired. However, excessively rapid heating should be avoided becauseit may cause peeling and crazing of the coating.

Following drying, the coating is fired to a temperature sufficient tocause the fusion and interaction thereof with the glass substrate. lhave found that this can conveniently be accomplished with the air of anatural gas-oxygen flame without the need for heating the entire glassarticle to the temperature at which the coating will fuse to form avitreous layer. Hence, sufficient heating to cause reaction of thecoating with the glass substrate to form the vitreous glaze can beobtained simply by directing a gas-oxygen flame onto the area of thecoating to be reacted for a period of several seconds or until thecoating has been converted to the vitreous state as the result ofinteraction with the highsilica glass substrate.

The effectiveness of glazes provided according to the described methodin inhibiting devitrification of highsilica glasses was determined by aseries of experiments wherein several Corning Code 7913 (96 percentsilica) glass plates, each having a portion of its surface protected bya glaze, were subjected to a highly alkaline atmosphere at elevatedtemperatures. The composition of the protective glazes varied fromsample to sample so that the effect of coating composition on theprotective qualities of the glazes could be judged. The test conditionscomprised placing each partially-coated sample over a cruciblecontaining 1 gram of sodium carbonate so that the sample acted as acover having a portion of its surface protected by the glaze to betested. Each crucible, cover, and contents were then placed in afurnace, heated to a temperature of 1,000C., and maintained at thattemperature for 16 hours. At the end of this test period, the furnaceand contents were cooled and the cover plates were removed and examinedfor devitrification. In each case, the unprotected portions of the coverplates were severely devitrified, while the protected portions exhibitedvarying degrees of devitrification depending upon the protectivecapabilities of each composition. Representative results of these testsare set forth in Table I below, which lists the results for severaldifferent coating compositions and compares the degree ofdevitrification observed in the uncoated portion with that observed inthe glazed portion in each case. The degree of devitrification wasjudged on a qualitative basis, with extensive surface crystallizationand crazing being termed severe, and light surface crystallizationtypified by a hazy surface appearance being termed slight. In the caseof coatings within the range of compositions of the invention, nosurface haze or crystallization could be detected in the glazed surfaceregions of any of the samples.

TABLE I Surface DevitrificationCode 7913 Plates SodiumRich Atmosphere 16Hours at 1.000" C.

Devitrification Unglazed Glazed Sample Coating Composition Region Region100% Ta O, severe none 2 60% Ta,0,40% SiQ, severe none 3 50% Ta,O,-50%Ago. severe none 4 30% Ta,O,-% Al O, severe none 5 20% Ta O,% M 0,severe none 6 10% Ta,O M 0, severe slight 7 Al,0; severe slight Tap-containing glazes were also applied to vitreous fused silica and othersilicate glasses with good results.

In one series of tests, a coating consisting essentially of 50 percentTa O and 50 percent SiO by weight was applied to portions of thesurfaces of both vitreous fused silica plates and plates of aborosilicate glass containing about 80 percent silica by weight, andreacted to form a glaze. These plates were then subjected to testconditions similar to those described above, except that a temperatureof 800C. rather than l,000C. was maintained during the Na,CO exposureperiod. In both cases, the glazed surface regions of the test plateswere completely protected from devitriflcation, whereas the unglazedsurface regions exhibited moderate to severe devitrification.Accordingly, it has been concluded that the glazes of the presentinvention are suitable for providing some degree of devitriflcationprotection to most silicate glasses under highly alkaline conditions atelevated temperatures.

The invention may be further understood by reference to the followingdetailed examples, which illustrate the best mode contemplated by theinventor for carrying out his invention. I

EXAMPLE I A 400 ml. beaker containing 94 ml. of B was placed under apropeller-type variable-speed mixer and stirring was started at a mediumspeed. Approximately 3.5 grams of powdered alumina, having particlesizes in the range from about 0.01-0.05 microns, was slowly added to thewater in the beaker while stirring was continued. Next, approximately2.5 grams of TaCl was added to the mixture while stirring was continued.This quantity of TaCl will yield approximately 1.5 grams of Ta O uponfiring, so that the final coating was calculated to contain about 70percent A1 0 and 30 percent Ta O by weight. The pH of the slurry wasadjusted to about 9.0 through the addition of NH OI-I, and the mixer wasthen turned off. A 1% inch square Corning Code 7913 glass plate about 4millimeters in thickness and consisting of about 96 percent silica byweight was then partially dipped in the slurry for about 30 seconds,removed, and air dried until visible moisture was no longer evident. Itwas then further dried with a yellow gas flame, having a temperatureestimated to be about 650C. Following drying, the coating was reactedwith the substrate by heating with a gas-oxygen flame. The

coating was completely transformed into a vitreous layer by flametreating for a few seconds. After the treatment, the coated and uncoatedmaterials could not be distinguished by visual inspection.

was subjected along with an unglazed sample having the same compositionto a devitrification test as hereinbefore described, wherein each samplewas placed over a crucible, each crucible, plate and contents thenheated to a temperature of l,000C. in a furnace, and maintained at thattemperature for 16 hours. Following this treatment the samples werecooled and examined, and it was determined that, while the unglazedplate had severely devitrified, the plate protected by the glazeprepared according to Example I was completely vitreous and suffered noapparent surface crystallization.

EXAMPLE III A slurry was prepared according to the procedure outlined inExample I, except that 4 grams of powdered silica, having an averageparticle size of about 0.01 microns, and 6 grams of Ta O weresuccessively added to ml. of H 0, so that the final coating wouldcontain 60 percent Ta O and 40 percent SiO by weight. The pH of thisslurry was adjusted to about 8.0 through the addition of NH OI-I. AComing Code 7913 glass plate was coated with the slurry, dried, andfired as described in Example I. Both glazed and unglazed samples werethen subjected to devitrifying conditions as described in Example II,and subsequently examined. It was found that, while the unglazed platehad severely devitrified, the plate protected by the protective glazeapplied as described was completely vitreous, exhibiting no visiblesurface crystallization.

From the above examples, it has been concluded that the protectiveglazes of the invention offer a useful solution to the problem ofsurface devitrification of highsilica glasses under alkaline conditionsat elevated temperatures.

I claim:

1. A high-silica glass article coated with a devitrification-resistantglaze formed by firing onto said article a coating consistingessentially, in weight percent on the oxide basis, of about l5-l00percent Ta O 0-85 percent Al o and 0-50 percent'SiO,

2. A high-silica glass article according to claim 1 wherein said coatingconsists essentially, in weight percent on the oxide basis, of about55-65 percent 'Ia O and 35-45 percent $0,, and wherein said glaze has athickness in the range from about 10-50 microns.

3. A high-silica glass article according to claim I wherein said coatingconsists essentially, in weight percent on the oxide basis, of about25-35 percent Ta,O and 65-75 percent A1 0 and wherein said glaze has a

2. A high-silica glass article according to claim 1 wherein said coatingconsists essentially, in weight percent on the oxide basis, of about55-65 percent Ta2O5 and 35-45 percent SiO2, and wherein said glaze has athickness in the range from about 10-50 microns.
 3. A high-silica glassarticle according to claim 1 wherein said coating consists essentially,in weight percent on the oxide basis, of about 25-35 percent Ta2O5 and65-75 percent Al2O3, and wherein said glaze has a thickness in the rangefrom about 10-50 microns.